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Practice Exam III Fall 2004

# Practice Exam III Fall 2004 - 1 Rank the following from...

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1) Rank the following from smallest radius to largest radius (no explanation required) Ar, S, S -2 , Ga +3 SMALLEST LARGEST Ga +3 Ar S S -2 2) What is the molality of a solution of sugar (C 6 H 12 O 6 ) dissolved in ethanol (CH 3 CH 2 OH) which has a vapor pressure of 745 torr at the normal boiling point of ethanol? Since we are at the normal boiling point the vapor pressure P° = 760 745 = X ethanol 760 0.9803 = moles ethanol / (mol ethanol + mole sugar) Assume that we have 1 kg of ethanol: 1000g x 1 mol/46g = 21.74 moles ethanol 0.9803 = 21.74/(21.74+x) x = 0.437 moles sugar m = 0.437 mol/1 kg solvent = 0.437 m 3) Below is the phase diagram for Helium. Respond to the following questions: A) A student claims to have generated a new technology based on the sublimation of He. Can this student be right and if so, what pressure would need to be applied to observe this? No, there is no T/P at which solid and vapor are in equilibrium B) Another student claims to have developed a new technology based on the liquid-liquid transition at 1 atm. Can this student possibly be correct and if so at what temperature would this be observed? Yes, about 2.2 K C) A student claims to have started with He gas at 5.5 K and cooled at a constant pressure of 1.0 atm and have formed solid He. Describe what happens to He gas as it is cooled from 5.5 K to 0.001 K at 1 atm and decide whether the student is correct. It starts as a gas and quickly condenses into liquid I and then liquid II, but never solid! 1

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4) Calculate the pH of each of the following solutions a) 0.023 M H 2 SO 4 [H 3 O + ] = 0.023, pH = 1.64 b) A mixture of 100.00 mL each of 0.05 M NaOH and 0.05 HCl These will neutralize (equal amounts of OH - and H + ) pH = 7 c) 0.05 M NH 4 Cl (K b for NH 3 is 1.85 x10 -5 ) NH 4 + is conjugate acid of NH 3 so K a = K w /K b = 10 -14 /1.85x10 -5 = 5.41x10 -10 5.41x10 -10 = x 2 /(.05-x) x = 5.2x10 -6 pH = 5.28 2
5) For the following BALANCED endothermic reaction Fe(s) + 4H 2 O(g) 4H 2 (g) + Fe 3 O 4 (s) a) Assuming that the reaction is AT equilibrium, what will be the effect of each of the following CHANGES: More P, Less P, No change are your choices: i. Increasing the pressure by decreasing the volume No effect (equal number of moles on each side) ii. Addition of more iron NO effect (Fe is a solid and does not appear in the K expression) iii. Addition of more water Since H 2 O is a GAS, it DOES appear in the K expression so adding water would lead to more P. iv. Placing the reaction mixture (i.e., the beaker) into an ice bath Since the reaction is ENDOthermic heat can be considered one of the reactants and placing the mixture in an ice pabth would REMOVE heat (i.e., remove reactant) so LESS P. 6) What is the boiling point of a solution of aqueous HI (HI is a strong acid) with a pH of 1.270, assuming the density of the solution is 1.090 g/mL? [H 3 O + ] = 10 -1.27 = 0.0537 M = [HI] Since the density is 1.09 g/mL we know that 1 liter would have a mass of 1090 grams and could contain 0.0537 moles of HI.

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Practice Exam III Fall 2004 - 1 Rank the following from...

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